The long-range goal of this project is to define the mechanism of phosphoinositide 3-kinase (PI3K) signaling in lymphocytes. As PI3K is required for lymphocyte proliferation, advances in this area may lead to novel strategies for the treatment of immunodeficiency, autoimmunity, transplant rejection and cancer. The central hypothesis guiding this application is that the PI3K regulatory isoforms p85alpha and p85beta have distinct functional roles in lymphocyte signaling. To test this hypothesis, this investigator?s laboratory has generated mice lacking either p85alpha or p85beta. p85alpha-deficient mice exhibit B cell defects similar to those seen in mice lacking Btk, BLNK or PLCgamma2. These findings support a model that PI3K activation is important for membrane assembly of a signaling complex that facilitates PLCgamma2 activation and sustained calcium flux. T cells and B cells rely on distinct proteins to carry out many of the signaling steps that link the antigen receptor to calcium flux and proliferation. For example, calcium flux in T cells is regulated by Itk, a functional homolog of Btk. Preliminary results suggest that p85beta may be an important PI3K regulatory isoform in T cell signaling. This application has three specific aims: Aim 1 is to test the model that p85alpha is required for activation of Btk and PLCgamma2 leading to sustained Ca2+ flux. Biochemical assays will be used to analyze signaling in primary B cells and immortalized B cell lines lacking p85alpha. Aim 2 is determine if p85beta is required for T cell proliferation and cytokine production. Aim 3 is to determine whether p85beta is required for activation of Itk, PLCgamma1, and sustained Ca2+ flux in T cells. Biochemical assays will be used to analyze signaling in purified T cells. These studies will increase our understanding of PI3K signaling, a central control point in lymphocyte proliferation.